The present invention relates to a display-integrated type tablet device for use in a personal computer, a word processor, or the like.
As a means for inputting a handwritten letter or a figure into a computer or a word processor, there has been put into practical use a display-integrated type tablet device which is formed by laminating an electrostatic induction type tablet on a liquid crystal display and is capable of receiving an input of a letter or a figure into its electrostatic induction type tablet as if the letter or figure were written on a paper by writing implements. However, in such a display-integrated type tablet device, electrodes are viewed as a grating on the display screen due to a difference in reflectance or transmittance between a portion having an electrode and a portion having no electrode, which has been a cause of degrading the quality of an image displayed on the liquid crystal display screen.
As a tablet free of the above-mentioned drawback, lately a display-integrated type tablet device as shown in FIG. 8 is proposed (Japanese Patent Application No. 3-46751 and a copending U.S. patent application No. 07/849 733). It should be noted that the device of FIG. 8 was invented by one of the inventors of this invention, and was yet not open when this invention was made, and therefore is not a prior art.
In the above-mentioned display-integrated type tablet device, electrodes concurrently serve as image display electrodes of a liquid crystal display (LCD) and as coordinate detection electrodes of an electrostatic type tablet device. There are provided in one frame period a coordinate detection period when designated coordinates on the tablet are detected and an image display period when an image is displayed as shown in FIG. 9 to time-sharingly effect the coordinate detection and image display.
Referring to FIG. 8, an LCD panel 1 is constructed by interposing liquid crystals between common electrodes Y.sub.1 through Y.sub.n (an arbitrary common electrode represented by Y hereinafter) and segment electrodes X.sub.1 through X.sub.m (an arbitrary segment electrode represented by X hereinafter) which are arranged at right angles to each other, in which each portion where a common electrode Y and a segment electrode X cross each other constitutes each pixel. In other words, n.times.m dot pixels are arranged in matrix in the LCD panel 1.
The above-mentioned display-integrated type tablet device has an advantage of permitting cost reduction as well as compact and light weight design by virtue of the concurrent use of the electrodes and drive circuits as those of the liquid crystal display and those of the electrostatic induction type tablet in addition to an advantage of making the grating-shaped electrodes invisible for a better image presentation in contrast to the conventional type formed by laminating the electrostatic induction type tablet on the liquid crystal display.
The above-mentioned display-integrated type tablet device operates as follows. A common electrode drive circuit 2 for driving the common electrode Y and a segment electrode drive circuit 3 for driving the segment electrode X are connected to a display control circuit 5 and a detection control circuit 6 via a switching circuit 4. The switching circuit 4 is controlled by a control circuit 7 so that it outputs an output signal from the display control circuit 5 to the common electrode drive circuit 2 and the segment electrode drive circuit 3 in an image display period or outputs an output from the detection control circuit 6 to the common electrode drive circuit 2 and the segment electrode drive circuit 3 in a coordinate detection period.
Although the switching circuit 4, the display control circuit 5, the detection control circuit 6, and the control circuit 7 are expressed dividedly in blocks in FIG. 8, the circuits are integrated in an LSI (Large Scale Integrated) circuit in practice. Therefore, the LSI cannot be strictly sectioned into such blocks in a practical circuit arrangement.
In the above-mentioned image display period, there are output, from the display control circuit 5, shift data s from a shift data output terminal S, an inverted signal fr from an inverted signal output terminal FR, a clock signal cp1 from a clock output terminal CP1, a clock signal cp2 from a clock output terminal CP2, and display data D.sub.0 through D.sub.3 from data output terminals D0 through D3.
The above-mentioned clock signal cp1 is a clock signal which has a period when pixels in one line display an image, and the signal is input as a clock signal cp1o to a clock input terminal YCK of the common electrode drive circuit 2 and a latch pulse input terminal XLP of the segment electrode drive circuit 3 via an output terminal CP1O of the switching circuit 4. The shift data s which is a pulse signal for selecting a specified common electrode Y is input as shift data so to a shift data input terminal DIO1 of the common electrode drive circuit 2 in synchronization with the clock signal cp1o via an output terminal SO of the switching circuit 4.
When the shift data so is input to the common electrode drive circuit 2, the pulse position of the shift data so is shifted in a shift register in synchronization with the clock signal cp1o, and drive pulses of a common electrode drive signal are applied to the common electrodes Y.sub.1 through Y.sub.n from output terminals O1 through On of the common electrode drive circuit 2 in correspondence with the shift position. The common electrode drive signal is generated based on bias power sources V.sub.0 through V.sub.5 supplied from a DC power supply circuit 12.
The above-mentioned clock signal cp2 is a clock signal which has a period being a division of a period when pixels in one line displays an image, and the signal is input as a clock signal cp2o to a clock input terminal XCK of the segment electrode drive circuit 3 via an output terminal CP2O of the switching circuit 4.
The image display data D.sub.0 through D.sub.3 are input as display data D.sub.0 o through D.sub.3 o to input terminals XD0 through XD3 of the segment electrode drive circuit 3 via output terminals D0O through D3O of the switching circuit 4, and then successively taken into a register in the segment electrode drive circuit 3 in synchronization with the clock signal cp2o. When all the image display data corresponding to the pixels in one line are taken in, the display data taken in are latched at a timing of the clock signal cp1o input to the latch pulse input terminal XLP. Then drive pulses of the segment electrode drive signal corresponding to the display data are applied from output terminals O1 through Om of the segment electrode drive circuit 3 to the segment electrodes X.sub.1 through X.sub.m. The segment drive signal is also generated based on the bias power sources V.sub.0 through V.sub.5 supplied from the DC power supply circuit 12.
It is noted that the inversion signal fr is a signal for preventing the possible deterioration of the liquid crystals due to electrolysis by periodically inverting the direction of voltage application to the liquid crystals in the image display period. The inversion signal fr is input as an inversion signal fro to an inversion signal input terminal YFR of the common electrode drive circuit 2 and an inversion signal input terminal XFR of the segment electrode drive circuit 3 via an inversion signal output terminal FRO of the switching circuit 4.
Thus the pixel matrix of the LCD panel 1 is line-sequentially driven by the operations of the above-mentioned common electrode drive circuit 2 and the segment electrode drive circuit 3 to display an image corresponding to the display data D.sub.0 through D.sub.3 on the LCD panel 1.
In the aforementioned coordinate detection period, there are output, from the detection control circuit 6, shift data sd from a shift data output terminal Sd, an inversion signal frd from an inversion signal output terminal FRd, a clock signal cp1d from a clock output terminal CP1d, a clock signal cp2d from a clock output terminal CP2d, and drive data D.sub.0 d through D.sub.3 d from data output terminals D0d through D3d.
The clock signal cp1d is a clock signal which has a period when one common electrode Y is scanned, and the signal is input as the clock signal cp1o to the clock input terminal YCK of the common electrode drive circuit 2 and the latch pulse input terminal XLP of the segment electrode drive circuit 3 via the output terminal CP1O of the switching circuit 4. Meanwhile, the shift data sd which is a pulse signal for selecting a specified common electrode Y is input as the shift data so to the shift data input terminal DIO1 of the common electrode drive circuit 2 via the output terminal SO of the switching circuit 4 in synchronization with the aforementioned clock signal cp1d.
Then, in the same manner as described above, the pulse position of the shift data so is shifted in the shift register of the common electrode drive circuit 2 in synchronization with the clock signal cp1o, and scanning pulses of common electrode drive signals Y.sub.1 through Y.sub.n (arbitrary common electrode scanning signal represented by y hereinafter) are successively applied from the output terminals O1 through On corresponding to the shift position to the common electrodes Y.sub.1 through Y.sub.n. The common electrode scanning signal y is generated based on the bias power sources V.sub.0 through V.sub.5 supplied from the DC power supply circuit 12.
The above-mentioned clock signal cp2d is a clock signal which has a period when the segment electrode X is scanned, and the signal is input as the clock signal cp2o to the clock input terminal XCK of the segment electrode drive circuit 3 via the output terminal CP2O of the switching circuit 4.
The drive data D.sub.0 d through D.sub.3 d are input as drive data D.sub.0 o through D.sub.3 o to the input terminals XD0 through XD3 of the segment electrode drive circuit 3 via the output terminals D0O through D3O of the switching circuit 4, and then successively taken into the register of the segment electrode drive circuit 3 in synchronization with the clock signal cp2o. Then scanning pulses of the segment electrode scanning signals x.sub.1 through x.sub.m (arbitrary segment electrode scanning signal represented by x hereinafter) corresponding to the above-mentioned drive data are output from the output terminals O1 through O.sub.m of the segment electrode drive circuit 3 to segment electrodes X.sub.1 through Xm. The segment electrode scanning signal x is also generated based on the bias power sources V.sub.0 through V.sub.5 supplied from the DC power supply circuit 12.
FIG. 10 is a timing chart of the scanning signals in the coordinate detection period of the above-mentioned display-integrated type tablet device. The coordinate detection period is separated into an x-coordinate detection period and a subsequent y-coordinate detection period. In the x-coordinate detection period, the segment electrode scanning signal x which is a pulse voltage signal is sequentially applied to the segment electrode X. In the y-coordinate detection period, the common electrode scanning signal y which is a pulse voltage signal is sequentially applied to the common electrode Y.
With the application of the above-mentioned pulse voltage signals, a voltage is induced at a designation coordinate detection pen (referred to merely as the "detection pen" hereinafter) 8 due to a floating capacity between the segment electrode X or the common electrode Y and a tip electrode of the detection pen 8. The voltage induced at the detection pen 8 is amplified in an amplifier 9 and then input to an x-coordinate detection circuit 10 and a y-coordinate detection circuit 11.
The-x-coordinate detection circuit 10 and the y-coordinate detection circuit 11 detect an x-coordinate value or a y-coordinate value of a position designated by the detection pen 8 by detecting a period from the time when the pulse voltage signal is applied to the time when an induction voltage takes its maximum value based on an output from the amplifier 9 and a timing signal from the control circuit 7.
In the above case, the width of the scanning pulse applied to the segment electrode X or the common electrode Y (referred to merely as the "scanning electrode" hereinafter) is determined by giving shift data s.sub.0 having a high logic level "H" to an input terminal of the shift register in the first stage for a period from the time of starting the detection to the time when pulses are shifted in the shift register by a specified frequency so that an optimum pulse width is achieved with respect to a filter coefficient and position detection accuracy in the analog data processing of the detection signal induced at the detection pen 8. In other words, the scanning pulse has a width corresponding to the width of a plurality of electrode lines, the width being equal to the pulse width of the shift data so.
It is assumed for simplicity of description hereinafter that the common electrode Y is located in an upper position and the segment electrode X is located in a lower position.
The above-mentioned display-integrated type tablet device, however, has the following problems.
&lt;First problem&gt;
The circuit composed of the scanning electrodes and the common electrode drive circuit 2 or the segment electrode drive circuit 3 (referred to merely as the "driver" hereinafter) is idealistically a lumped parameter circuit. Therefore, when a voltage is applied to a scanning electrode by the driver, it is natural that the entire scanning electrode to which the voltage is applied is to have an identical application voltage. Consequently, for example, on an identical common electrode Y, an identical y-coordinate value is to be obtained when whatever portion of the electrode is designated by the detection pen 8.
However, since the resistance of the scanning electrode and the capacitance between the upper and lower scanning electrodes constituting each pixel cannot be ignored, the above-mentioned circuit is assumed to be a distributed parameter circuit. Therefore, a propagation delay of the application voltage takes place in the scanning electrodes. The propagation delay takes place as displacement of the detection coordinate in the scanning direction. Furthermore, the propagation delay increases in quantity as the designated portion is being away from the driver as shown in FIG. 11, and therefore the detection coordinate differs depending on the distance from the position designated by the detection pen 8 to the driver on an identical electrode.
&lt;Second problem&gt;
As described above, the scanning pulse has a pulse width corresponding to the plural electrode lines. In such a case, taking the x-coordinate as an example, the voltage induced at the detection pen 8 is laterally symmetrical about its peak position in a central portion in the x-direction, where the peak position coincides with the designation position of the detection pen 8 as shown in FIG. 12. However, since there is no segment electrode X outside the image display area where both the scanning electrodes exist, the number of segment electrodes X to which is applied a voltage reduces in a peripheral area in the x-direction of the designation position. Therefore, the voltage induced at the detection pen 8 is laterally asymmetrical about the peak position, and the peak position is displaced inward from the designation position.
As a result, there is a relation between the designation position in the x-direction and the detection x-coordinate value in the central area in y-direction as shown in FIG. 13, and therefore the correct detection coordinate cannot be obtained due to the inward displacement of the detection coordinate in the peripheral area in the x-direction.
&lt;Third problem&gt;
In the time of scanning the lower electrode when viewed from the coordinate detection surface, there is an electrostatic coupling between the lower electrode and the upper electrode in addition to the electrostatic coupling between the lower electrode and the tip electrode of the detection pen 8 in the strict sense of the word, and therefore a voltage is induced at the upper electrode. As a result, in the time of scanning the lower electrode, there is executed a coordinate calculation based on a detection signal formed through superimposition of the detection signal induced at the tip electrode of the detection pen 8 due to the electrostatic coupling between the upper electrode being not scanned and the detection pen electrode on the regular detection signal.
In the above-mentioned case, the scanning pulse has a width corresponding to the width of the plural electrode lines, and therefore a scanning pulse is applied to a certain number of scanning electrodes. Therefore, at the time of starting and ending the scanning of the lower electrode, the number of electrodes to which is applied the voltage among all the lower electrodes changes (i.e., the number increases at the starting time and reduces at the ending time). The above also causes a change of voltage induced at the upper electrodes, which results in increasing the ratio of a component attributed to the induction voltage at the upper electrodes with respect to a component attributed to the scanning of the lower electrodes in the detection signal.
Since the upper electrodes cover the entire scanning area of the lower electrodes, a detection signal having a peak other than the peak corresponding to the regular designation position of the detection pen is output regardless of the position of the detection pen as shown in FIG. 14 at the time of starting and ending the scanning of the lower electrodes, which is an obstacle in detecting the detection pen position through scanning the lower electrodes.
In other words, at the designation position where the peak corresponding to the regular designation position of the detection pen and the other peaks are superimposed (i.e., in the peripheral area of the display screen), the superimposition waveform is distorted to fail in obtaining the correct detection coordinates.
&lt;Fourth problem&gt;
As illustrated in FIG. 15, the scanning electrodes are arranged in parallel in the image display area, however, the electrodes are convergently connected to a driver IC (Integrated Circuit) having a narrow width constituting the aforementioned driver at the end portions. Therefore, a variation in distribution density of the electrodes exists in an area outside the image display area. Since plural number of such ICs are arranged, the above-mentioned distribution density variation pattern corresponds in number to the ICs.
FIG. 16 shows a relation between an x-direction designation position and a detection x-coordinate value in the peripheral area in y-direction (i.e., in the vicinity of the segment drive circuit). Referring to FIG. 16, when no variation in distribution density of the electrodes exists, the detection x-coordinate value has a linear relation to the actual x-direction designation position as indicated by the dotted line. However, actually a curve containing a superimposed periodical fluctuation corresponding to the variation in distribution density of the electrodes as indicated by the solid line results failing in obtaining a correct detection coordinate value.
The number of crests of the fluctuation waveform is equal to the number of the driver ICs, where one period coincides with each of the intervals between the driver ICs. The amplitude of the fluctuation waveform reduces as the designation position is being away from the driver ICs.